A technique of evaluating a state of deterioration a battery and its remaining lifetime by analysis of impedances of the battery has hitherto been proposed.
For instance; Patent Document 1, describes a method for measuring a state of an electrical discharge of a battery having internal impedance. The method includes measuring a first internal impedance of the battery at a first frequency; measuring a second internal impedance of the battery at a second frequency; determining a difference between the first internal impedance and the second internal impedance; and determining a deflection angle or an argument that represents the difference between the internal impedances; in other words, a deflection angle or an argument that represents a state of an electrical discharge of the battery. Further, the first frequency or the second frequency, whichever is lower, exists between 0.1 Hz and 10 Hz.
Patent Document 2 includes a description about detecting a quantity of electricity associated with impedance of a rechargeable battery or a quantity of electricity associated with maximum output density by application of an AC voltage to the rechargeable battery and determining performance of the rechargeable battery on the basis of the quantity of electricity. Another description also states that the quantity of electricity is an AC-impedance-associated electrical quantity associated with, of impedances of the rechargeable battery, an AC impedance component made up of a component which varies according to a frequency of an AC voltage. Still another description states that a real-axis component value and an imaginary-axis component value of an impedance of the rechargeable battery are determined for each frequency value by applying AC voltages of a plurality of frequency values in a predetermined frequency band to the rechargeable battery and that a quantity of electricity associated with impedance is calculated from the real-axis component value and the imaginary-axis component value. A further additional description states that an AC impedance component is calculated from a circular locus of impedance in a two-dimensional plane defined by an axis representing the real-axis component values and another axis representing the imaginary-axis component values.